Question

A satellite in a stable orbit contains two closed vessels: one of these is filled with water while the other is filled with hot steam. Why does the water exert very little pressure on its container but the steam exerts almost the same as it wouldon earth?

Answer 1

I am not sure that my answer will satisfy you but here I am...

The steam is hot but what does hot means? It means that the molecules of your gas have energy (to do things such as move around) so that, apart for being in the gaseous state with molecules free to wander around, every molecule has energy to travel around inside the vessel; but it is like to have a room full of people bouncing around! They run in every direction (they are free as a gas) and have energy to run around (the heat that makes your gas hot). They keep on bouncing, in particular against the walls of your vessel producing PRESSURE (if you replace one of the walls of your vessel with a piston the molecules will collide with the surface of the piston and push it showing PRESSURE!!!).

Water is more fixed; our persons/molecules are now connected (by the hands, for example) they still have a kind of mobility but they cannot wander around freely as a in a gas and there is now no gravity to produce pressure, say, on the bottom of your vessel. On Earth water would "weight" and produce pressure on the surface of the bottom of the vessel (if the bottom of your vessel is not strong enough it could break and let water spill).

Gravity on Earth would affect your gas molecules as well but singularly and, because of this, weakly while in a liquid a molecule at the "bottom" will "feel" all the other molecules on top of her that stays there due to the "connection" of the structure of liquid water. In orbit the effect of gravity would be dramatically reduced so that now your liquid molecule at the bottom does not feel such a strong effect of the presence of her sisters on top of her.

So

hot steam = a bunch of fast moving molecules bouncing around and colliding with the walls (in orbit as on Earth).

Liquid water = molecules joined together and not so free to bounce around and, in orbit, free from the effect of “weighting” on one surface of your vessel.

Hope it is not too confusing!